The present invention relates to a process for removing nonmetallic impurities from metallurgical silicon.
The term “metallurgical silicon” used in this document is intended to encompass all Si quality grades, i.e. metallurgical silicon, UMG (upgraded metallurgical-grade) silicon, solar-grade silicon and electronic-grade silicon, and also the associated raw silicon and the associated pre-products of the Si quality grades and of the corresponding raw silicon. The term “metallurgical silicon” is therefore to be interpreted in the broadest sense.
Metallurgical silicon is purified by means of different technologies in order to obtain raw silicon which is suitable for the production of solar cells. Metallurgical silicon treated in this way is referred to as UMG (upgraded metallurgical-grade) silicon. UMG raw silicon often still has relatively high concentrations of impurities, which are undesirable and have to be removed in order to be able to produce high-quality solar cells. German patent application 10 2008 025 263.8 describes a process for purifying metallurgical silicon, in which halide-containing silicon is added to metallurgical silicon, a melt is produced from the substances and the impurities are sublimated out of the melt and removed in the form of metal halides. This process therefore relates to the removal of metals from the metallurgical silicon. The process is also suitable for further purifying UMG raw silicon. Metals removed include metalloids, alkaline earth metals, etc. It has also been observed that very good results can be obtained with this process for pre-products of UMG raw silicon. Pre-products of this type are likewise referred to as UMG raw silicon in the following.
With respect to the removal of nonmetallic impurities from silicon, a process for removing phosphorus impurities is known from U.S. Pat. No. 4,312,849, for example. In this case, a silicon melt is produced, and a gas containing a chlorine source is blown through said melt.
In another known process for removing phosphorus from a silicon melt, the melt is subjected to a vacuum treatment. Yet another known process relates to the simultaneous removal of boron and phosphorus by means of a plasma purification process.
The removal of nonmetallic impurities from metallurgical silicon with the aid of gaseous chlorine sources is therefore already known. In this case, gas mixtures containing chlorine gas or chlorine are introduced into the Si melt. The implementation of such technology is very complex, however, since the chlorine has to be introduced directly into the melt, which is generally effected by means of small tubes or special nozzles. Therefore, a homogeneous distribution of the chlorine over the entire melt is only possible to a limited extent. Moreover, the apparatuses for introducing the chlorine into the melt can adversely affect the melt itself, that is, impurities originating from the apparatuses for introducing gas can occur, for example.